Frame synchronization is very important to packet data transmission in OFDM or OFDMA communication systems. In an OFDM or OFDMA-based communication system, such as IEEE 802.16d/e system, the apparatus of frame synchronization is responsible for detecting the beginning instant of a frame. With the beginning instant of the frame detected, it is possible to feed the preamble symbol and the following symbols into a Fast Fourier Transform (FFT) module for further processing in accordance with a fixed symbol length. On the other hand, most receivers require an automatic frequency correction (AFC) unit to correct carrier frequency offset (CFO) between the mobile station (MS) and the base station (BS). AFC can be realized by adjusting a voltage-controlled oscillator (VCO) or a numeral-controlled oscillator (NCO). As described below, adjustment of CFO affects the performance of frame synchronization.
FIG. 1 shows a typical structure of a frame of the TDD-OFDM/OFDMA system, which frame includes a downlink (DL) sub-frame and an uplink (UL) sub-frame. A receive/transmit transition gap (RTG) separates the UL sub-frame from the DL sub-frame, and a transmit/receive gap (TTG) separates the DL sub-frame from the UL sub-frame. The first symbol of a frame is a training symbol called “preamble”, which is a sign of the beginning of the frame. The preamble symbol is transmitted in every frame, and its power is specified several dB higher than that of a normal data symbol. The common symbol and power boosting properties of the preamble symbol are contributive to realization of frame synchronization.
FIG. 2 shows a conventional apparatus of frame synchronization based on the preamble symbol.
As shown in FIG. 2, two adjacent frames are cross-correlated in a delay correlator 202, and a correlation peak as obtained corresponds to the beginning position of the preamble symbol. However, adjustment of an AFC unit to the frequency will affect the correlation value of the preamble symbol.
Suppose the receiving signal of a previous frame be r(n−Nframe), wherein n is the sequence number of a time domain sample, and Nframe is the number of samples in one frame duration; let a normalized frequency offset be δf1={tilde over (δ)}f1/Δf, wherein Δf is subcarrier spacing and {tilde over (δ)}f1 is CFO between MS and BS. This receiving signal can be modeled as Equation (1).r(n−Nframe)=r′(n−Nframe)e−j2πnδf1/Nfft  (1)
where r′(n) is the received signal without frequency offset from the transmitter, Nfft is the FFT window size of the OFDM signal.
Suppose the frequency offset estimated in a frequency offset estimator 206 be {circumflex over (δ)}f1, AFC acts before the current frame comes, that is to say, the frequency offset is compensated in a frequency offset compensator 201. The residual frequency offset is as follows:δf2=δf1−{circumflex over (δ)}f1  (2)
This results in a frequency offset difference between the two frames. The receiving signal of the current frame can be modeled as Equation (3).r(n)=r′(n)e−j2πnδf2/Nfft  (3)
The cross-correlation of the two frames is given by Equation (4) below:
                                                                                          P                  1                                ⁡                                  (                  n                  )                                            =                                                                                    ∑                                          k                      =                      1                                                              N                      sym                                                        ⁢                                                                          ⁢                                                            r                      ⁡                                              (                                                  n                          +                          k                                                )                                                              ⁢                                                                  r                        *                                            ⁡                                              (                                                  n                          +                          k                          -                                                      N                            frame                                                                          )                                                                                                                                                                                                                                          =                                                                                                                                                ∑                                                          k                              =                              1                                                                                      N                              sym                                                                                ⁢                                                                                                          ⁢                          r                                                ’                                            ⁢                                              (                                                  n                          +                          k                                                )                                            ⁢                      r                                                        ’                                ⁢                                  (                                      n                    +                    k                    -                                          N                      frame                                                        )                                ⁢                                  ⅇ                                                            -                      j                                        ⁢                                                                                  ⁢                    2                    ⁢                                          π                      ⁡                                              (                                                                              δ                            ⁢                                                                                                                  ⁢                                                          f                              1                                                                                -                                                      δ                            ⁢                                                                                                                  ⁢                                                          f                              2                                                                                                      )                                                              ⁢                                          k                      /                                              N                        fft                                                                                                                                                                  (        4        )            
where n is the running index of the time domain sample corresponding to the output correlation value, k is the index of the sample for the correlation calculation in an OFDM symbol, and correlation window length is a symbol, namely Nsym.
If without consideration of CFO difference between two successive frames, there will always be a correlation peak at the beginning of the preamble symbol due to similarity of the received preamble symbol. However, every item in Equation (4) carries a time variant phase rotation e−j2π(δf1−δf2)k/Nfft introduced by CFO difference. The phase rotation is changed as k changing, which turns a near in-phase summation into a vector summation when the preamble symbol is fed into the correlation window. The correlation peak of the preamble symbol is not prominent especially when CFO is big. Accordingly, it is difficult to search the beginning of the frame in this case, that is, false timing probability will increase.
This problem can be solved by stopping AFC adjustment during the frame timing period. However, an additional feedback control circuit from the timing synchronization apparatus is required and much synchronization time is consumed.
On the other hand, RTG and TTG in the TDD-OFDM/OFDMA system tend to lead to false frame detection with conventional methods. In conventional methods correlation value P1(n) is usually normalized by the corresponding symbol power P2(n) to eliminate the channel fading influence. The power of the current symbol is calculated in a power calculator 203 by the following Equation:
                                          P            2                    ⁡                      (            n            )                          =                                                      ∑                              k                =                1                                            N                sym                                      ⁢                                                  ⁢                                          r                ⁡                                  (                                      n                    +                    k                                    )                                            ⁢                                                r                  *                                ⁡                                  (                                      n                    +                    k                                    )                                                                                                  (        5        )            
P1(n) is normalized by P2(n) in a normalizer 204, that is:
                              ɛ          ⁡                      (            n            )                          =                                            P              1                        ⁡                          (              n              )                                                          P              2                        ⁡                          (              n              )                                                          (        6        )            
The normalizing power P2(n) is very small within or around RTG/TTG. By this time, at the ending terminal or the beginning terminal of the UL/DL sub-frame, the correlator 202 combines several samples of an end symbol with RTG or TTG together as one OFDM symbol to perform the correlation calculation. It is easy for several residual samples to be closely correlated. Thus, the difference between P1(n) and P2(n) is very small, and this results in the fact that the normalized ε(n) around RTG/TTG is often bigger than the normalized correlation value of the preamble symbol. This phenomenon is most notable in RTG period because the power of UL received by a mobile station may be tens of dB higher than the power of DL when a neighboring user is transmitting signals.
Because of the same reason, the false correlation peak also tends to appear in null symbols of a frame where no data transmission is scheduled or around symbols of a frame where the power of transmission is boosted or decreased.